Tools: Force small lib when compiling with uARM

[theotherjimmy]

Description

PR #7197 blurred the lines between the ARM and uARM compilers. When making
those changes, I forgot to force -t uARM to compile with microlib. This PR
corrects the bad behavior that I introduced then

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[theotherjimmy]

@adbridge @0xc0170 This fixes the bug i introduced into Mbed2 builds.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2363
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7231/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : FAILURE

Build number : 1988
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7231/

[mbed-ci]

Test : SUCCESS

Build number : 2151
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7231/2151

[0xc0170]

/morph export-build

[0xc0170]

#7197 (comment) - With this fix, now a target like nucleo f207 will be tested for mbed 2 with uARM (default toolchain). As we found out the last week. just want to make sure we do not have this again.

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1992
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7231/